Reduced iron powder and method for preparing same and bearing

ABSTRACT

Reduced iron powder that has fewer coarse inclusions, has excellent formability, has high porosity after sintering, has excellent reactivity per unit mass, and can be effectively used as reaction material even to the particle inside is provided. Reduced iron powder has an apparent density of 1.00 Mg/m 3  to 1.40 Mg/m 3 .

TECHNICAL FIELD

The disclosure relates to reduced iron powder, a method for preparingthe same, and a bearing produced from the reduced iron powder.

BACKGROUND

Two main types of iron powder based on its preparation method aretypically known: reduced iron powder; and atomized iron powder. Theapparent density of iron powder currently known is 2.3 Mg/m³ or more inreduced iron powder, and 2.5 Mg/m³ or more in atomized iron powder. Thespecific surface area of iron powder is 0.10 m²/g or less in reducediron powder, and 0.07 m²/g or less in atomized iron powder.

Iron powder having such characteristics has many uses, and particularlyits uses in chemical reaction material, sintered machine parts, etc.make up a high proportion. In chemical reaction material, large specificsurface area is required for efficient reaction. In sintered machineparts, high porosity is required as oilless bearings (which is alsoreferred to as “oil retaining bearings”).

The specific surface area is larger when the apparent density is lower.Iron powder with low apparent density is needed to produce sinteredmachine parts with high porosity.

As an example of sintered machine parts, an oilless bearing is describedbelow. It is important that the oilless bearing maintains appropriateoil content. If the oil content is low, adequate lubricity anddurability cannot be obtained. To maintain appropriate oil content, thesintered body needs to be increased in porosity. JP 2001-132755 A(PTL 1) describes a relevant technique.

With reduction in size of machine parts, oilless bearings ofapproximately 2 mm in outer diameter and 0.6 mm in inner diameter havebeen produced in recent years. However, the use of conventional reducediron powder for smaller parts causes poor formability and poor yieldrate because conventional reduced iron powder has coarse pores and ironportions, making production difficult. This has increased demand foriron powder that is finer in microstructure, is more porous, and hasfewer inclusions than conventional iron powder.

If a part requires contacting with another part as in the case of abearing, the presence of inclusions in the part damages the other partand shortens the life of the product. Besides, in the case where theinclusions do not sinter with the surrounding iron powder, theinclusions cause structural defects. This significantly decreases theyield rate or the strength, particularly when producing small machineparts.

The “inclusions” mentioned here has the following meaning. Reduced ironpowder is produced from iron ore or mill scale. The purity of thereduced iron powder as the product is determined by the purity of theiron oxide as the raw material. The most common impurity is oxygen.Oxygen mostly appears as a thin film of surface oxide. Basic impuritiesinclude carbon, magnesium, aluminum, silicon, phosphorus, sulfur,chromium, manganese, nickel, and copper. Many of these impurities arepresent as oxides, and are called inclusions.

For use in chemical reaction material, iron powder with large specificsurface area, i.e. low apparent density, is known to be useful asdescribed in JP 4667835 B2 (PTL 2) and JP 4667937 B2 (PTL 3), given thatlarger specific surface area of powder contributes to more efficientreaction.

CITATION LIST Patent Literatures

PTL 1: JP 2001-132755 A

PTL 2: JP 4667835 B2

PTL 3: JP 4667937 B2

SUMMARY Technical Problem

In the case of using conventional reduced iron powder to produce abearing, the shaft is damaged or the bearing develops structural defectsbecause the reduced iron powder contains inclusions exceeding 200 μm.

Besides, in the production of bearings, there is a possibility that thecirculation performance of lubricating oil cannot be obtained because,with reduction in size of bearings, pores or iron microstructure becomeslarge relative to a bearing as mentioned above. In other words, althoughconventional reduced iron powder has fine pores, its many inclusionscause the product to fail. Bearings with inner diameter of 0.6 mm andouter diameter of 2.0 mm can be produced at a relatively high yield rateeven when conventional reduced iron powder is used. In the case ofproducing smaller bearings, for example, with inner diameter of 0.4 mmand outer diameter of 1.4 mm using conventional reduced iron powder,however, formability is insufficient and the yield rate dropssignificantly, making mass production difficult.

Atomized iron powder is not suitable for use in the aforementioned smallbearings, as its smooth surface causes insufficient bonding powerbetween iron powder particles during forming and leads to asignificantly lower rattler value. Moreover, in the production ofoilless bearings, atomized iron powder has a major drawback of havingfew pores and hindering sufficient circulation of oil. Atomized ironpowder is also problematic in that it has few fine pores, although itsinclusions are few.

From the perspective of using reduced iron powder as chemical reactionmaterial, the powder is required to have large specific surface area sothat the powder has excellent reactivity per unit mass and even theparticle inside can be effectively used as reaction material.

As described above, reduced iron powder whose apparent density is muchlower than 2.0 Mg/m³ and whose specific surface area is 0.2 m³/g ormore, which is much higher than 0.1 m³/g, is needed in order to producebearings with inner diameter of less than 0.6 mm and outer diameter ofless than 2.0 mm at a high yield rate. Such reduced iron powder,however, cannot be prepared by conventional production methods.

It could therefore be helpful to provide reduced iron powder that hasfewer coarse inclusions, has excellent formability, has high porosityafter sintering, has excellent reactivity per unit mass, and can beeffectively used as reaction material even to the particle inside, amethod for preparing the same, and a bearing produced from the reducediron powder.

Solution to Problem

We Thus Provide:

1. Reduced iron powder having an apparent density of 1.00 Mg/m³ to 1.40Mg/m³.

2. The reduced iron powder according to 1., having an amount of oxygenof 0.38 mass % or less.

3. The reduced iron powder according to 1. or 2., having a specificsurface area of 0.20 m²/g or more.

4. A method for preparing reduced iron powder, for use in preparing thereduced iron powder according to any one of 1. to 3., comprising:agglomerating precursor iron oxide powder whose mean particle sizemeasured by a laser diffraction method is 3.0 μm or less to obtain ironoxide powder; and thereafter reducing the iron oxide powder at 800° C.to 1000° C. with hydrogen to obtain the reduced iron powder.

5. The method for preparing reduced iron powder according to 4.,comprising classifying and selecting the iron oxide powder so that itsmean particle size measured by the laser diffraction method is 50 μm to200 μm, before the reduction of the iron oxide powder.

6. The method for preparing reduced iron powder according to 4. or 5.,wherein the iron oxide powder has an iron content of 68.8 mass % ormore.

7. A bearing produced from the reduced iron powder according to any oneof 1. to 3. as a raw material.

Advantageous Effect

It is thus possible to obtain reduced iron powder that has fewer coarseinclusions, has excellent formability, has high porosity aftersintering, has excellent reactivity per unit mass, and can beeffectively used as reaction material even to the particle inside.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a flow diagram illustrating a process of preparing reducediron powder according to one of the disclosed embodiments; and

FIG. 2 is a diagram illustrating an appearance image and cross-sectionalimage of each of a conventional example and Examples 1 and 2.

DETAILED DESCRIPTION

We succeeded in producing new reduced iron powder having an apparentdensity of 1.00 Mg/m³ to 1.40 Mg/m³ and a specific surface area of 0.20m ²/g or more by a new preparation method. The reduced iron powderaccording to the disclosure has sufficiently low apparent density, andtherefore has excellent formability, has excellent reactivity per unitmass, and can be effectively used as reaction material even to theparticle inside. The reduced iron powder according to the disclosurealso has fine iron microstructure (see the white portions in thecross-sectional images in FIG. 2), as a result of which inclusions arefinely dispersed. Hence, the reduced iron powder can be used as rawmaterial to produce high-strength bearings at a high yield rate. Forexample, bearings with inner diameter of 0.4 mm and outer diameter of1.4 mm can be mass-produced at a high yield rate.

The following describes a method for preparing reduced iron powderaccording to one of the disclosed embodiments, with reference to FIG. 1.First, iron oxide powder (precursor iron oxide powder) having apredetermined mean particle size is agglomerated to obtain iron oxidepowder. The obtained iron oxide powder is then classified and selectedso that its mean particle size is set to a predetermined range. Afterthis, the iron oxide powder is reduced with hydrogen gas(Fe₂O₃+3H₂=2Fe+3H₂O) and crushed as appropriate to obtain reduced ironpowder (porous iron powder).

It is important to refine the precursor iron oxide powder as startingmaterial so that its mean particle size (D50) measured by a laserdiffraction method is 3.0 μm or less, in order to set the apparentdensity of the reduced iron powder to 1.40 Mg/m³ or less to thus makethe inclusions in the reduced iron powder finer. The refinement makespores smaller, which contributes to finer inclusions. The mean particlesize of the precursor iron oxide powder is preferably 2.0 μm or less. Nolower limit is placed on the mean particle size of the precursor ironoxide powder, yet in industrial terms the lower limit is approximately0.5 μm.

An example of the method for preparing the precursor iron oxide powderis a method of neutralizing and extracting waste acid after picklingsteel sheets in a steelworks. For example, a method using a sprayroasting furnace by the Ruthner process and a fluidized roasting methodby the Lurgi process are available.

It is essential to agglomerate the precursor iron oxide powder to obtainiron oxide powder formed by the coagulation of the precursor iron oxidepowder. Effective methods of agglomerating the precursor iron oxidepowder include a method of mixing a binder and water into the precursoriron oxide powder using a Henschel mixer and drying the mixture, and amethod of dissolving the precursor iron oxide powder in water togetherwith a binder to form slurry and then drying the droplets with hot air(spray dryer). In both methods, the binder may be PVA, starch, or thelike.

When a vessel or a reducing furnace is charged with the iron oxidepowder to reduce the iron oxide powder, voids formed between coagulatedparticles ensure appropriate air permeability, thus facilitating thereduction. To achieve this, the mean particle size of the iron oxidepowder after the agglomeration is important. Moreover, the particle sizeof the iron oxide powder to be reduced correlates with the particle sizeof the reduced iron powder. It is therefore preferable to classify andselect the iron oxide powder after the agglomeration to control its meanparticle size, before reducing the iron oxide powder.

The mean particle size of the iron oxide powder after the agglomerationis important, as mentioned above. However, not all particles necessarilymaintain their shape. For example, a plurality of particles may bondwith each other, or one particle may be broken. Accordingly, we madecareful examination, and discovered that the mean particle size of thereduced iron powder effective in practical terms is 50 μm to 100 μm and,to achieve this, the mean particle size of the iron oxide powder ispreferably 50 μm to 200 μm. Thus, it is preferable to appropriatelyclassify and select the iron oxide powder after the agglomeration sothat its mean particle size is set to 50 μm to 200 μm.

It is also preferable that the iron content in the iron oxide powder is68.8 mass % or more. This sufficiently reduces the amount of oxygen inthe reduced iron powder, and further enhances the effect of improvingchemical reactivity and the effect of producing high-strength bearingsat a high yield rate. No upper limit is placed on the iron content inthe iron oxide powder, yet the upper limit is approximately 77 mass %.

The iron oxide powder after the agglomeration is reduced to obtainreduced iron powder (also simply referred to as iron powder). Wediscovered the conditions for preparing iron powder that has lowapparent density, i.e. approximately half that of conventional reducediron powder or atomized iron powder, and in which inclusions are finelydispersed, by appropriately managing the reduction temperature in thisreduction step which is hydrogen reduction of iron oxide. It isimportant to set the reduction temperature during the reduction to 800°C. or more and 1000° C. or less. If the reduction temperature is lessthan 800° C., it is difficult to remove oxygen in the reduced ironpowder by reduction reaction. As a result, a large amount of oxygenremains in the iron powder. This causes insufficient chemical reactivityand decreases formability, and leads to a lower yield rate in theproduction of bearings. If the reduction temperature is more than 1000°C., the sintering of the iron powder progresses and the apparent densityexceeds 1.40 Mg/m³. This causes insufficient chemical reactivity, andleads to a lower yield rate in the production of bearings.

The reduction time is preferably 120 min or more, to sufficiently reduceiron oxide powder yielded from fine precursor iron oxide powder of 3.0μm or less in mean particle size to obtain reduced iron powder of 1.00Mg/m³ to 1.40 Mg/m³ in apparent density. No upper limit is placed on thereduction time, yet the upper limit may be approximately 240 min interms of process efficiency.

The conditions other than the reduced iron powder preparation conditionsdescribed above may be well-known reduced iron powder preparationconditions. An example of the reduction method is a method of heatingiron powder at atmospheric pressure using a belt furnace or the like ina reducing atmosphere such as hydrogen.

The following describes reduced iron powder according to one of thedisclosed embodiments. The reduced iron powder has an apparent densityof 1.00 Mg/m³ to 1.40 Mg/m³, and can be prepared by the preparationmethod described above for the first time. If the apparent density ofthe reduced iron powder is less than 1.00 Mg/m³, the specific surfacearea is excessively large, which increases the risk of a dust explosion,i.e. rapidly progressing reaction with oxygen in the air. If theapparent density of the reduced iron powder is more than 1.40 Mg/m³,chemical reactivity is insufficient. Besides, the strength of the greencompact decreases. This facilitates failures in subsequent steps, andleads to a lower yield rate in the production of bearings.

When the apparent density of the reduced iron powder is in the range of1.00 Mg/m³ to 1.40 Mg/m³, the green strength increases, and bearings canbe produced at a high yield rate. Moreover, by limiting the apparentdensity to this range, coarse inclusions are effectively reduced, andthe strength after sintering is improved, thus contributing to higherbearing quality. Further, the reduced iron powder has excellentreactivity per unit mass, and can be effectively used as reactionmaterial even to the particle inside. The apparent density is measuredaccording to JIS-Z-2504.

The amount of oxygen in the reduced iron powder is preferably 0.38 mass% or less. This further enhances the effect of improving chemicalreactivity and the effect of producing high-strength bearings at a highyield rate. No lower limit is placed on the amount of oxygen in thereduced iron powder, yet the lower limit is approximately 0.10 mass %.

If the specific surface area of the reduced iron powder is less than0.20 m²/g, iron powder particles characteristic of the disclosure arenot formed sufficiently, leading to insufficient chemical reactivity.The specific surface area of the reduced iron powder is thereforepreferably 0.20 m²/g or more. No upper limit is placed on the specificsurface area of the iron powder, yet the upper limit is preferablyapproximately 0.4 m²/g in terms of handling and the like. The specificsurface area is measured by a BET method using nitrogen gas.

A bearing can be produced from the reduced iron powder as raw material.The bearing has an excellent yield rate in bearing production andexcellent strength and porosity, and has high chemical reactivity, asdescribed in the following examples. The method for producing thebearing from the reduced iron powder as raw material may be aconventional method except that the reduced iron powder is used as rawmaterial.

EXAMPLES

Table 1 compares conventional reduced iron powder (reduced iron powderobtained through two reduction steps), conventional atomized ironpowder, and reduced iron powders (Comparative Examples 1 to 5, Examples1 to 4) obtained through the preparation process illustrated in FIG. 1.In Comparative Examples 1 to 5 and Examples 1 to 4, hydrogen was used asreducing gas. The conventional reduced iron powder was prepared asfollows: Using iron ore or mill scale as raw material, coke powder wasadded and primary reduction using a tunnel furnace was performed withoutan agglomeration step and a classification step in FIG. 1, and thenreduction in the thick-line box was performed.

The iron powder evaluation items listed in Table 1 were evaluated asfollows.

The mean particle size of precursor iron oxide powder was measured by avolume-based laser diffraction method.

The iron content in iron oxide powder was measured according toJIS-M-8212.

The mean particle size of iron oxide powder after agglomeration wasmeasured by a laser diffraction method, and set as 50% particle size.

The apparent density of reduced iron powder was measured according toJIS-Z-2504.

The mean particle size of reduced iron powder was measured by avolume-based laser diffraction method, and set as 50% particle size.

The specific surface area of reduced iron powder was measured by a BETmethod using nitrogen gas.

The amount of oxygen in reduced iron powder was measured by an inert gasfusion infrared absorption method (GFA).

The yield rate in bearing production was evaluated as pass when thefailure rate from the green compacting in the shape of a cylinder withan inner diameter of 0.4 mm, an outer diameter of 1.4 mm, and a heightof 2 mm to 2.5 mm to the completion of sintering was 5% or less (a yieldrate of 95% or more). The strength was evaluated as pass when thestrength upon compressing the cylinder in a lying state was 17 N/mm² ormore, and fail when the strength was less than 17 N/mm².

The porosity is a factor determining the performance of an oillessbearing, and its appropriate value is 18% to 22%.

The porosity was measured by mercury porosimetry.

The reaction rate in chemical reaction was evaluated based on thereaction in which sulfur content in soil adsorbed to iron (Fe+S=FeS).Adsorptivity by this reaction is required to be a predetermined level ormore in practical terms. Accordingly, in Table 1, chemical reactivity isset as an index represented by a ratio to 1 as the minimum requiredlevel.

FIG. 2 illustrates an appearance image and cross-sectional image of thereduced iron powder of each of Examples 1 and 2, in comparison with theconventional reduced iron powder. The appearance image was taken using ascanning electron microscope (SEM), and the cross-sectional image wastaken using an optical microscope. Many pores were contained insideparticles in Examples 1 and 2, as compared with the conventional reducediron powder.

TABLE 1 Preparation conditions for reduced iron powder Characteristicsof reduced iron powder Mean particle size Iron content Mean particleMean Specific of precursor iron in iron oxide size of iron oxideReduction Reduction Apparent particle surface Amount of oxide powderpowder powder time temperature density size area oxygen (μm) (mass %)(μm) (min) (° C.) (Mg/m³) (μm) (m²/g) (mass %) — — — — — 2.2 to 55 to0.07 to <0.40 2.7 105 0.10 — — — — — 2.5 to 50 to 0.04 to <0.30 3.1 900.08 2.8 68.8 120 240 1050 1.48 100 0.20 0.22 2.8 68.8 120 240 780 0.9880 0.31 0.40 3.2 68.8 150 240 850 0.95 90 0.28 0.45 2.8 68.8 45 240 8501.49 60 0.22 0.21 2.8 68.8 220 240 850 0.95 150 0.33 0.55 2.8 68.8 50240 1000 1.38 80 0.22 0.25 2.8 68.8 120 240 1000 1.32 80 0.25 0.27 2.868.8 120 240 800 1.03 60 0.28 0.36 2.8 68.2 110 240 850 1.12 80 0.250.43 0.7 69.0 90 240 850 1.05 75 0.30 0.37 Characteristics in bearingproduction Chemical Yield rate Strength Porosity reactivity (%) (N/mm²)(%) (—) Remarks 84 15 18 0.7 Conventional reduced iron powder 62 9 240.5 Conventional atomized iron powder 92 18 20 0.8 Comparative Example 192 19 21 0.9 Comparative Example 2 92 19 20 0.9 Comparative Example 3 8816 16 0.8 Comparative Example 4 92 19 26 0.8 Comparative Example 5 96 2122 1.4 Example 1 98 25 21 1.3 Example 2 98 24 20 1.3 Example 3 95 17 191.2 Example 4 97 23 22 1.3 Example 5

Comparative Example 1 is iron powder obtained by reducing iron oxidepowder at 1050° C. Its apparent density was 1.48 Mg/m³, which is outsidethe range according to the disclosure. While the degree of reduction wasrelatively favorable, the yield rate in bearing production was evaluatedas fail. The chemical reactivity was also evaluated as fail.

Comparative Example 2 is iron powder obtained by reducing iron oxidepowder at 780° C. Its apparent density was 0.98 Mg/m³, which is outsidethe range according to the disclosure. The yield rate in bearingproduction and the chemical reactivity were evaluated as fail.

Comparative Example 3 is iron powder obtained using precursor iron oxidepowder of 3.2 μm in mean particle size and by reducing iron oxide powderafter agglomeration at 850° C. Its apparent density was 0.95 Mg/m³,which is outside the range according to the disclosure. The degree ofreduction was relatively low. The yield rate in bearing production waspoor, and the chemical reactivity was evaluated as fail.

Comparative Example 4 is reduced iron powder obtained using iron oxidepowder after agglomeration of 45 μm in mean particle size. Its apparentdensity was 1.49 Mg/m³, which is outside the range according to thedisclosure. The degree of reduction was high, and the strength of thebearing was evaluated as pass. Meanwhile, the yield rate in bearingproduction was evaluated as fail. The chemical reactivity was alsoevaluated as fail.

Comparative Example 5 is reduced iron powder obtained using iron oxidepowder after agglomeration of 220 μm in mean particle size. Its apparentdensity was 0.95 Mg/m³, which is outside the range according to thedisclosure. The strength of the bearing was evaluated as pass, but theyield rate in bearing production was evaluated as fail. The porosity wasexcessive, and the chemical reactivity was evaluated as fail.

Example 1 is iron powder obtained using iron oxide powder afteragglomeration of 50 μm in mean particle size and by reducing the ironoxide powder after agglomeration at 1000° C. Its apparent density was1.38 Mg/m³. The degree of reduction was high, and the yield rate inbearing production and the strength and porosity of the bearing were allevaluated as pass. The chemical reactivity was also favorable.

Example 2 is iron powder obtained using iron oxide powder afteragglomeration of 120 μm in mean particle size and by reducing the ironoxide powder after agglomeration at 1000° C. Its apparent density was1.32 Mg/m³. The degree of reduction was favorable, and the yield rate inbearing production and the strength and porosity of the bearing were allevaluated as pass. The chemical reactivity was also favorable.

Example 3 is iron powder obtained using iron oxide powder afteragglomeration of 120 μm in mean particle size and by reducing the ironoxide powder after agglomeration at 800° C. Its apparent density was1.03 Mg/m³. The degree of reduction was favorable, and the yield rate inbearing production and the strength and porosity of the bearing were allevaluated as pass. The chemical reactivity was also favorable.

Example 4 is iron powder obtained using iron oxide powder afteragglomeration having an iron content of 68.2 mass %. Its apparentdensity was 1.12 Mg/m³, while the amount of oxygen in the reduced ironpowder was 0.43 mass %. The chemical reactivity was favorable, and theyield rate in bearing production and the strength and porosity of thebearing were all evaluated as pass.

Example 5 is iron powder obtained using precursor iron oxide powder of0.7 μm in mean particle size, with the mean particle size of iron oxidepowder being 90 μm. Its apparent density was 1.05 Mg/m³. The chemicalreactivity was favorable, and the yield rate in bearing production andthe strength and porosity of the bearing were all evaluated as pass.

1. Reduced iron powder having an apparent density of 1.00 Mg/m³ to 1.40Mg/m³.
 2. The reduced iron powder according to claim 1, having an amountof oxygen of 0.38 mass % or less.
 3. The reduced iron powder accordingto claim 1, having a specific surface area of 0.20 m²/g or more.
 4. Amethod for preparing reduced iron powder, for use in preparing thereduced iron powder according to claim 1, comprising: agglomeratingprecursor iron oxide powder whose mean particle size measured by a laserdiffraction method is 3.0 μm or less to obtain iron oxide powder; andthereafter reducing the iron oxide powder at 800° C. to 1000° C. withhydrogen to obtain the reduced iron powder.
 5. The method for preparingreduced iron powder according to claim 4, comprising classifying andselecting the iron oxide powder so that its mean particle size measuredby the laser diffraction method is 50 μm to 200 μm, before the reductionof the iron oxide powder.
 6. The method for preparing reduced ironpowder according to claim 4, wherein the iron oxide powder has an ironcontent of 68.8 mass % or more.
 7. A bearing produced from the reducediron powder according to claim 1 as a raw material.
 8. A bearingproduced from the reduced iron powder according to claim 2 as a rawmaterial.
 9. A bearing produced from the reduced iron powder accordingto claim 3 as a raw material.